This invention relates to a method and apparatus for indirect calorimetry employing respiratory gas analysis and more particularly to a method and system which determines the oxygen and/or carbon dioxide content of the expired gas using measurements of mass and volume of the expired gas and mass and volume of the inspired gas as measured by transit time of ultrasonic pulses passed through the gas.
I have a number of patents on respiratory calorimeters. Some of these operate by integrating the flow volume of a number of inhalations and exhalations over a period of time and by subtracting the CO2 volume in the exhalation from the integral of the exhaled volume by scrubbing the CO2 and then subtracting the exhaled flow volume less the CO2 volume from the inhaled flow volume to determine oxygen consumption during the period. I also have a pending application that measures both inspired and expired volume and either O2 or CO2 content to determine oxygen consumption. The carbon dioxide scrubber is bulky and requires replenishment after a number of uses. Carbon dioxide or oxygen analyzers are also relatively expensive.
It has previously been proposed to determine the mass of a gas flowing through a conduit by determining the transit time of ultrasonic pulses passed through the gas in a direction having a component along the axis of flow so as to determine the flow rate of the gas, and additionally determining the density of the gas. U.S. Pat. No. 2,911,825 discloses such a system in which the acoustic impedance of the gas is measured to determine the density. U.S. Pat. No. 5,214,966 similarly employs the transit time of ultrasonic pulses to determine the flow rate and determines the density of the flowing gas through measurement of the velocity of sound through the gas. U.S. Pat. No. 5,645,071 uses the transit time of ultrasonic pulses to determine the flow rate and additionally makes temperature measurements which, with the flow rate, allow the determination of mass of the flowing gas. This latter patent also suggests the application of this device to pulmonary function diagnostics and discloses an additional gas analyzing sensor for determining the carbon dioxide and/or oxygen content of the flowing gas on an on-line, real time basis.
It would be desirable to provide a method of analysis which allows the determination of oxygen consumption, carbon dioxide production and related and derived respiratory factors without the need for any gas analyzers, such as O2 and CO2 analyzers. This would result in a low cost, high precision instrument suitable for a wide range of health care applications.
Accordingly, the present invention is directed toward a method and apparatus for analyzing respiratory gases to determine oxygen consumption for indirect calorimetry purposes as well as CO2 production and related respiratory factors, by measuring the mass and flow volume of expired gas without the need for additional analysis of the oxygen or CO2 content of the expired gas, through use of measurements of the inhaled gas. In its simplest form, in which the constituents of the inhaled gas are known with sufficient precision, as is the case when the subject is breathing ambient air, and the temperature and humidity of the inspired and expired gases are the same as a result of passage through an artificial nose or the like, or are measured or assumed, the O2 and CO2 contents of the exhaled gases may be determined from measurements of the inhaled and exhaled flow volumes and the mass of the exhaled gases. Alternatively, the mass of the inhaled gas will also be measured. The measurements are preferably made by a subject breathing through the apparatus of the present invention for five to ten minutes with the measurements of the inhalations and exhalations being integrated over those periods.
To understand the method of the present invention and the system for implementing it, assume that the subject is breathing ambient air which has a composition of 79% nitrogen, 21% oxygen and 0.03% CO2. By measuring the flow volume of the inhalations over the test period, the inhaled mass may be determined. Assuming that the exhalations are at the same humidity and temperature as the inhalations, from measurements of the integrated mass and flow volume of the exhalations the CO2 and O2 contents of the exhalations may be determined since the nitrogen content of the inhalations and exhalations will be the same, leaving only two unknowns, and after equalization for the differential in volumes between the inhaled gas the exhaled gas, the mass of the exhaled gas will vary linearly as a function of its CO2 and O2 content. The determination of the O2 and CO2 content of the expired volume is possible because CO2 has a substantially higher density than O2 and moles of O2 and CO2 occupy the same volume so that substitution of CO2 in the exhaled gas for O2 in the inhaled gas changes the gas mass but not the volume.
The system of the present invention preferably makes the flow measurements of the inhaled and exhaled volumes with known ultrasonic pulse transit time techniques and determines gas density with measurements such as acoustic impedance, speed of sound, or temperature. The same apparatus can measure the masses and flow volumes of the inhaled and exhaled gases.